Participants are screened and evaluated for neuropsychological impairment. Pending completion of a neurological examination, safety labs and meeting inclusion criteria criteria including demonstrated neropsychological impairment as defined by the protocol, patients are then randomized to either Lexipafant 250 mg po BID or placebo 250 mg po BID. Safety labs are repeated at screening, baseline and week 2, 6 and 10. Neuropsychological testing is done at screening, baseline and weeks 2, 6 and 10. Neuropsychological testing is done at screening and weeks 6 and 10. Neurological exam is performed at screening, baseline and week 10. After successful completion of the initial phase, participants can choose to go on the open label phase for 48 weeks, depending on the availability of the drug by the sponsor British Biotech. Patients come in for a final visit at the end of the open label for repeat safety labs, neuropsychological and neurological exams.